Querying and filtering business network data

Queries are used to return data about the blockchain world-state; for example, you could write a query to return all drivers over a specified age, or all drivers with a specific name. The composer-rest-server component exposes named queries via the generated REST API.

Queries are an optional component of a business network definition, written in a single query file (queries.qry).

Note: When using the Hyperledger Fabric v1.0 runtime Hyperledger Fabric must be configured to use CouchDB persistence.

Filters are similar to queries, but use the LoopBack filter syntax, and can only be sent using the Hyperledger Composer REST API. Currently, only the WHERE LoopBack filter is supported. The supported operators within WHERE are: =, and, or, gt, gte, lt, lte, neq. Filters are submitted using a GET call against an asset type, participant type, or transaction type; the filter is then supplied as a parameter. Filters return the results from the specified class, and will not return results from classes extending the specified class.

Types of Queries

Hyperledger Composer supports two types of queries: named queries and dynamic queries. Named queries are specified in the business network definition and are exposed as GET methods by the composer-rest-server component. Dynamic queries may be constructed dynamically at runtime within a Transaction Processor function, or from client code.

Writing Named Queries

Queries must contain a description and a statement. Query descriptions are a string that describe the function of the query. Query statements contain the operators and functions that control the query behavior.

Query descriptions can be any descriptive string. A query statement must include the SELECT operator and can optionally include FROM, WHERE, AND, ORDER BY, SKIP, and LIMIT.

Queries using the API

Queries can be invoked by calling the buildQuery or query APIs. The buildQuery API requires the entire query string to be specified as part of the API input. The query API requires you to specify the name of the query you wish to run.

Access Control for Queries

When returning the results of a query, your access control rules are applied to the results. Any content which the current user does not have authority to view is stripped from the results.

For example, if the current user sends a query that would return all assets, if they only have authority to view a limited selection of assets, the query would return only that limited set of assets.

Using filters

Filters can only be submitted using the Hyperledger Composer REST API, and must use the LoopBack syntax. To submit a query, a GET REST call must be submitted against an asset type, participant type, or transaction type with the filter supplied as a parameter. The supported data types for parameters to be filtered are numbers, Boolean, DateTime, and strings. A basic filter takes the following format, where op indicates an operator:

{"where": {"field1": {"op":"value1"}}}

Please note: Only the top level WHERE operator can have more than two operands.

Currently, only the WHERE LoopBack filter is supported. The supported operators within WHERE are: =, and, or, gt, gte, lt, lte, neq. Filters can combine multiple operators, in the following example, an and operator is nested within an or operator.

The between operator returns values between the given range. It accepts numbers, datetime values, and strings. If supplied with strings, the between operator returns results between the supplied strings alphabetically. In the example below, the filter will return all resources where the driver property is alphabetically between a and c, inclusively.